Insulated current sensor

ABSTRACT

A circuit for sensing a current comprises a substrate having a first and a second major surface, the second major surface being opposite to the first major surface. At least one magnetic field sensing element is arranged on the first major surface of the substrate and is suitable for sensing a magnetic field caused by a current flow in a current conductor coupled to the second major surface. The substrate also comprises at least one insulation layer, substantially buried between the first major surface and the second major surface of the substrate.

FIELD OF THE INVENTION

The present invention generally relates to current sensing devices andmethods for manufacturing thereof, and, more particularly, relates tocurrent sensing devices that are suitable for high current sensingapplications.

BACKGROUND OF THE INVENTION

Integrated circuits for measuring a current by means of sensing amagnetic field generated by a current flow are known in the field. Anadvancing degree of device miniaturization and cost reduction, however,leads to increased current densities, related ohmic losses, and heatdissipation that require careful consideration of the circuit design onthe one hand and a good trade-off between maximization of the sensedmagnetic field strength and sufficient voltage insulation between thecurrent carrying conductor and the sensing circuitry, as well asconnected external circuitry, on the other hand.

Both flip-chip assemblies implementing insulation barriers in a waferpostprocessing step and die-up configurations with insulation barriershave been described in prior art. The former solution needs aspecifically conceived redistribution layer to connect to the input andoutput connectors of the die to the pins of the package. Very thin leadfingers are required for a good input/output count, hence a customleadframe design and an extra etching step are provided for thisapproach, and a careful alignment of fingers belonging to a secondaryportion of the leadframe with respect to the flipped die establishes theelectrical coupling between die and leadframe. The latter solution ismore flexible having regard to the assembly of leadframe and die as wirebonding techniques can be used, but the signal weakness at the sensingelement is a disadvantage of die-up configurations. Good voltageinsulation can be achieved with ceramic interposers as insulationbarriers, but these are not readily available during waferpostprocessing but are provided only during assembly. Therefore, thereis a need for integrated current sensing circuits that provide goodvoltage insulation and signal strength, without adding additionalassembly steps.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide areliable current sensing device suitable for sensing high currents of upto at least 150 A (rms) and 200 A (peak) while withstanding andinsulating voltages up to at least 3 kV, and a method for manufacturingsuch current sensing device.

The above objective is accomplished by a device and method in accordancewith embodiments of the present invention.

In a first aspect, the present invention provides a circuit, e.g. anintegrated circuit, for sensing current. The circuit comprises:

-   -   a substrate having a first and a second major surface, the        second major surface being opposite to the first major surface;    -   at least one magnetic field sensing element arranged on said        first major surface of the substrate and suitable for sensing a        magnetic field caused by a current flow in a current conductor        coupled to the second major surface;    -   wherein the substrate comprises a first insulation layer        positioned between the first major surface and the second major        surface such that it is substantially buried in the substrate.        With the first insulation layer being substantially buried in        the substrate is meant that most, e.g. more than 60%, more than        80%, more than 90% or all of its outward directed substance is        interfacing a substrate material. In embodiments of the present        invention, the first insulation layer may be completely buried        in the substrate.

The first insulation layer may be fabricated during a wafer process,hence a cost-effective, reliable, and scalable, buried layer with highquality may be obtained. The first insulation layer can be thin. Therequired thickness of the buried insulation layer depends on thevoltages that will be applied and may correspond to about 1 μm per 500V.Hence for applications of about 2 kV, the insulation layer thickness canbe about 4 μm, but for applications up to 60V much lower insulationlayer thicknesses are needed. No extra die attach layer for aninterposer is required. Advantageously, strong currents are measurableat low risk.

In embodiments of the present invention, the at least one firstinsulation layer may be a buried insulating oxide layer or a buriedinsulating nitride layer. Silicon oxide, for example, has very goodinsulating properties. A nitride passivation layer also has goodinsulating properties, and moreover is good for moisture protection.

In a circuit according to embodiments of the present invention, thesubstrate may be an integrated sensor die.

In a circuit according to embodiments of the present invention,substrate material on either side of the first insulation layer may be asemiconductor material. In particular embodiments, the substrate may bea semiconductor-insulator-semiconductor (SOI) substrate, e.g. asilicon-insulator-silicon substrate, wherein the insulator forms thefirst insulation layer. It is an advantage to use such substrate, assemiconductor on insulator processing, e.g. silicon on insulatorprocessing, is a mature technique.

In a particular configuration, the SOI substrate may comprise asemiconductor layer, a buried insulation layer and an EPI layer. In theEPI layer, at least two Hall plates, or at least two groups of Hallplates with their outputs connected in parallel, may be provided asmagnetic field sensing element.

In embodiments of the present invention, the SOI substrate may becoupled to a reference potential, e.g. may be grounded, for instance bymaking use of a through-substrate via (TSV).

A circuit according to embodiments of the present invention may furthercomprise an external insulation layer positioned between the secondmajor surface and the current conductor. For instance, in case of an SOIsubstrate as indicated above, an external insulation layer, for instancea voltage insulation tape, may be provided between the semiconductorlayer of the SOI substrate and the current conductor. Such externalinsulation layer provides an additional protection for high currentapplications. Furthermore, such external insulation layer forms a secondreinforcement layer. It can be deposited on the substrate or on theleadframe, or it can be placed in between both, e.g. by attachmentlayers. A circuit according to embodiments of the present invention mayfor instance comprise an insulating tape coupled to the second majorsurface of the substrate. This insulating tape may be used to physicallyattach the substrate to a current conductor, such as for instance to theleadframe.

A circuit according to embodiments of the present invention may furthercomprise a leadframe comprising a primary and a secondary portion, theprimary and secondary portion each comprising a plurality of leads,wherein at least two of the leads of the primary portion of theleadframe are electrically connected to each other to form a currentconductor. Many leads may be connected to support current to bemeasured, thus wider connection pads may be obtained, having lowerresistivity and less heat dissipation.

In such a circuit according to embodiments of the present invention, atleast two magnetic field sensing elements may be provided, at least oneat each side of the current conductor, to measure the magnetic field.These sensors are advantageously used in inverted polarity so that anexternal magnetic field may be cancelled out.

In a circuit according to embodiments of the present invention, athickness of the substrate may be reduced in the vicinity of the atleast one magnetic field sensing element. Reducing the substratethickness decreases the distance between the current conductor and thesensing element for increasing sensor sensitivity, for instance forobtaining better SNR.

In a circuit according to embodiments of the present invention, a cavitymay be formed at the second major surface and in the vicinity of the atleast one magnetic field sensing element. This way, a locally thinmembrane may be formed for even more reduced distance and bettersensitivity, without, however, losing structural support. The cavity maycomprise chamfered side walls and/or straight side walls. The cavity maylock the conductor in place when aligning the leadframe with thesubstrate, without damaging the membrane.

In a circuit according to embodiments of the present invention, thefirst insulation layer may be partially exposed at the second majorsurface. This may be obtained by removing all the extra substrate at thebackside, even if wafer is already thinned.

In particular embodiments of the present invention, both with SOI-typesof substrate or with other substrates, the current conductor maycomprise a flat section and a protruding section, the flat sectionextending in a plane parallel to the second major surface, and theprotruding section extending out of said plane and into the cavity. Suchconductor geometry enables a closer placement of the conductor and thesensing element. It is easily obtainable by deformation or stamping.

In a circuit according to particular embodiments of the presentinvention, the die only overlaps the primary portion of the leadframe ina plan view parallel to the first major surface. This means that thesubstrate may be resting only on the primary portion of the leadframe,without overlapping the secondary portion of the leadframe in a planview parallel to the first major surface. This configuration enablessmaller dies resting on the primary leadframe portion, thus smaller costper die. Furthermore, longer primary/secondary distances are possible,which provides better insulation.

In a circuit according to embodiments of the present invention, the atleast one magnetic field sensing element may be electrically coupled tothe secondary portion of the leadframe, for instance via wire bonding.Wire bonding is a more flexible process than vias: wires are easier toplace and assemble, the alignment tolerance is much more relaxed, longerwires are available, and less die processing steps are required.

In a circuit according to embodiments of the present invention, thesubstrate may further comprise at least one second insulation layerpositioned between the first major surface and the second major surface,such that it is buried in the substrate and not touching the firstinsulation layer. This way increased insulating properties are obtained,for instance for high current applications and/or operatingvoltages/spikes. A substrate material located between the firstinsulation layer and the second insulation layer may be electricallygrounded. This grounded substrate material then forms an electrostaticshield, which avoids capacitive coupling of dV/dt transients from theprimary conductor.

In a circuit according to embodiments of the present invention, thesubstrate may further comprise an insulating circumferential walladjacent to its rim, the insulating wall extending transversely from thefirst insulation layer.

A circuit according to embodiments of the present invention may furthercomprise at least one flux concentrating magnetic layer arranged on thefirst major surface. This allows a local concentration of magnetic fieldlines, and thus a better sensitivity and SNR.

In a circuit according to embodiments of the present invention, the atleast one magnetic field sensing element may be implemented in a foreignmaterial which is electrically connected to the semiconductor materialof the substrate. This way, high responsivity materials may be used forthe sensing element.

In a circuit according to embodiments of the present invention, the atleast one magnetic field sensing element may be a printed magnetic fieldsensing element. This is an easy and low-cost method for integration ofmagnetic field sensing elements.

A circuit according to embodiments of the present invention may form anintegrated current sensor, whereby for instance all bits and pieces ofthe circuit are over-moulded in a single package.

In a second aspect, the present invention provides a method ofmanufacturing a device for measuring a current. The method comprises:

-   -   providing a semiconductor substrate having a first and a second        major surface, the second major surface being opposite to said        first major surface, and a first insulation layer which is, over        at least part of its surface, buried in the substrate between        said first and second major surface; and    -   providing at least one magnetic field sensing element on the        first major surface, the magnetic field sensing element being        suitable for sensing current flowing through a current conductor        physically coupled to the second major surface.

A method according to embodiments of the present invention may furthercomprise providing a current conductor and physically coupling thecurrent conductor to the second major surface.

In a method according to embodiments of the present invention, providinga current conductor may comprise:

-   -   providing a flat leadframe including a plurality of leads, of        which at least two leads are connected so as to form said        current conductor as a flat current conductor; and    -   etching a portion of the flat current conductor and/or using a        stamping tool to form a protruding section along the flat        current conductor.

It is an advantage of this embodiment that the leadframe can be thickfor better etching results.

A method according to embodiments of the present invention may furthercomprise the step of etching a cavity into the semiconductor substrate,the cavity being etched at the second major surface and in the vicinityof the at least one magnetic field sensing element. In accordance withembodiments of the present invention, etching can be wet or dry etchingfor controlling a side wall angle of the cavity. A method according toembodiments of the present invention may further comprise the step offilling the cavity with a low partial discharge susceptible siliconegel. The silicone may act as a sealing agent, as a thermal andelectrical insulator, and/or as an adhesive.

In a method according to embodiments of the present invention, couplingsaid current conductor to the second major surface may include applyinga conductive paste between the current conductor and the second majorsurface. This may provide improved adhesion, less heat, and/or a moreuniform current flow at target distance.

A method according to embodiments of the present invention may furthercomprise the step of applying, at a pre-determined distance from the rimof the semiconductor substrate, a deep trench insulation process to thesemiconductor substrate. This provides better intrinsic insulation withrespect to other circuitry, thus enabling formation of a larger and/ormore powerful device.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from thedependent claims may be combined with features of the independent claimsand with features of other dependent claims as appropriate and notmerely as explicitly set out in the claims.

For purposes of summarizing the invention and the advantages achievedover the prior art, certain objects and advantages of the invention havebeen described herein above. Of course, it is to be understood that notnecessarily all such objects or advantages may be achieved in accordancewith any particular embodiment of the invention. Thus, for example,those skilled in the art will recognize that the invention may beembodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

The above and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 shows a schematic cross-sectional view of a circuit suitable forsensing a current in accordance with an embodiment of the invention.

FIG. 2 illustrates an embodiment of the invention which includes twoinsulation layers, which are both substantially buried inside asubstrate and not in direct contact with each other.

FIG. 3 is a schematic perspective view of a part of an integratedcurrent sensing circuit in accordance with an embodiment of theinvention, wherein a cavity is formed at the second major surface.

FIG. 4 is a schematic cross-section along the line IV-IV of theembodiment shown in FIG. 3, wherein the cavity has straight side walls.

FIG. 5 shows a schematic cross-section similar to the one shown in FIG.4 but displays a cavity which has chamfered side walls.

The drawings are only schematic and are non-limiting. In the drawings,the size of some of the elements may be exaggerated and not drawn onscale for illustrative purposes. The dimensions and the relativedimensions do not necessarily correspond to actual reductions topractice of the invention.

Any reference signs in the claims shall not be construed as limiting thescope.

In the different drawings, the same reference signs refer to the same oranalogous elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings, but the invention isnot limited thereto but only by the claims.

The terms first, second and the like in the description and in theclaims, are used for distinguishing between similar elements and notnecessarily for describing a sequence, either temporally, spatially, inranking or in any other manner. It is to be understood that the terms soused are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other sequences than described or illustrated herein.

Moreover, directional terminology such as top, bottom, front, back,under, over and the like in the description and the claims is used fordescriptive purposes with reference to the orientation of the drawingsbeing described, and not necessarily for describing relative positions.Because components of embodiments of the present invention can bepositioned in a number of different orientations, the directionalterminology is used for purposes of illustration only, and is in no wayintended to be limiting, unless otherwise indicated. It is, hence, to beunderstood that the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the invention described hereinare capable of operation in other orientations than described orillustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, while some embodiments described herein include some, butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art.

It should be noted that the use of particular terminology whendescribing certain features or aspects of the invention should not betaken to imply that the terminology is being re-defined herein to berestricted to include any specific characteristics of the features oraspects of the invention with which that terminology is associated.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

A component or element arranged, provided, or formed on a surface, e.g.an electronic element arranged or formed on a substrate surface, mayeither be arranged, provided, or formed on top of that surface, thesurface being a support for the component or element, or may bearranged, provided, or formed in such a way that it is in contact withthat surface and is part of that surface but can, at the same time,extend into regions immediately below or above that surface.

In the context of the present invention, an insulation layer isconsidered substantially buried in a substrate if most of its outwarddirected substance is interfacing a substrate material. This includestwo cases: a first one in which an insulation layer is completely buriedin a substrate, meaning that all of its outward directed substance isinterfacing a substrate material, and a second one in which a smallamount of outward directed substance is interfacing a material not beingpart of the substrate. For the second case it is still true that where avolume of the substrate is defined, the insulation layer is buried inthis volume, meaning that it is interfacing a substrate materialeverywhere in this volume. A small volume of the substrate may beremoved such that another material is substituting the substratematerial, e.g. air in a cavity formed in the substrate. In the contextof the present invention, an insulation layer may be considered as beingsubstantially buried in a substrate if the perimeter of the insulationlayer is sandwiched within the substrate at both sides. In particularembodiments, an insulation layer may be considered as beingsubstantially buried in a substrate if more than 50%, for instance morethan 80% of its outward directed substance is interfacing a substratematerial.

An embodiment in accordance with a first aspect of the invention is nowdescribed with reference to FIG. 1, which is showing a schematiccross-sectional view of a circuit 100 suitable for sensing a current.The circuit 100, which is preferably an integrated circuit, comprises asubstrate 110 which may be coupled to a current conductor 101. Two majorsurfaces 116, 117, opposite to each other, are defined for the substrate110. A first major surface 116 of the substrate 110 may be a topsurface, and the second major surface 117 may be the bottom surface ofthe substrate 110. On, on top, or in the first major surface 116, atleast one magnetic field sensing element 113 is arranged at a straightdistance 109 from the current conductor 101, provided that the currentsensing circuit 100 is properly mounted thereon. The second majorsurface 117 may then be physically coupled to the current conductor 101;for the example shown in FIG. 1, a portion of the bottom side of thesubstrate 110 is attached to the current conductor 101 via an attachmentlayer 103. A first insulation layer 112 is substantially buried insidethe substrate 110. For this particular embodiment, the first insulationlayer 112 is completely buried in the substrate 110, meaning that thefirst insulation layer 112 is positioned between the first and thesecond major surfaces 116, 117 such that it is interfacing a substratematerial 111, 115 on either side.

Without being limited to an embodiment as shown in FIG. 1, a currentconductor 101 may be a portion of leadframe which provides the input andoutput terminals to the current sensing circuit 100, e.g. input andoutput access to a standard packaged integrated circuit, such as QFN,SOIC, etc., which are surface mountable to a printed circuit board andhave a small footprint. The leadframe may be implemented in copper, butother metals suitable for conducting a current are also usable. Forexample, the current conductor 101 may be formed as a primary portion ofthe leadframe, a secondary and other portions of the leadframe beingelectrically insulated from the primary portion for the packagedcircuit. In some embodiments of the invention, the leadframe comprises aplurality of leads in both a primary portion and a secondary portion102, and a current conductor 101 may be obtained by electricallyconnecting at least two of these leads belonging to the primary portion.This has the advantage that a contacting surface can be increased suchthat ohmic losses at this contact surface are minimized. It also reduceslocal heating of the contacting leads that form the current conductor101 and leads to a lower risk of heat induced failure of the currentsensing circuit 100. Especially in the case of miniaturized, integratedcurrent sensing circuits, high current densities are the result ofreduced cross-sectional areas of the leads or the current conductorinside the integrated circuit and elevated currents that are to bemeasured, e.g. currents higher than 1 A, higher than 30 A, higher than50 A, higher than 100 A, even up to 200 A, e.g. currents between 1 A and150 A, such as between 1 A and 50 A. The current conductor 101 may bephysically coupled to the substrate 110 in a direct or an indirectmanner. For some embodiments of the invention, an indirect coupling maybe provided by means of an attachment layer 103 as shown in FIG. 1. Anon-limiting example of an attachment layer 103 may be or may comprisean insulating tape, an adhesive film, an adhesive polymer film, or glue.In embodiments of the invention for which leads belonging to thesecondary portion 102 of the leadframe are used as output terminals, ofwhich FIG. 1 is an example, the at least one magnetic field sensingelement 113 is electrically coupled to one or more leads of the secondportion 102 of the leadframe. A suitable electrical coupling may beachieved by providing one or more bond pads 114 on the first majorsurface 116 of the substrate 110 and wire-bonding them, with a thin wireconnection 105, e.g. a thin gold wire, to the one or more leadsbelonging to the secondary portion 102. Long wire connections 105 arefeasible such that the substrate 110 is only resting on the primaryportion of the leadframe and therefore does not need to overlap, in aplan view parallel to the first major surface 116, with a secondaryportion 102 of the leadframe. Therefore smaller, more compact substrates110, e.g. smaller area semiconductor dies, may be used for an integratedcurrent sensing circuit which decreases the manufacturing cost perdevice. Moreover, it is possible to maintain a longer distance betweenthe primary and the secondary portion of the leadframe which improvesthe insulation properties of the circuit 100, for example largerclearance and creepage distances. The one or more bond pads 114 aretypically connected to outputs of signal processing circuitry on theactive side of the sensor substrate (e.g. semiconductor die) for furtherprocessing a signal transduced by the at least one magnetic fieldsensing element 113. The circuit 100 is typically an integrated,packaged one, meaning that the substrate 110, the leadframe, andpossible electrical connections between the substrate 110 and thesecondary portion 102 of the leadframe are encapsulated into a housing104. A housing 104 may be provided as a hardened polymer, e.g. asolidified thermoset polymer in an injection molding process for whichthe substrate 110, the leadframe, and possible electrical connectionsbetween the substrate 110 and the secondary portion 102 of the leadframeare placed into a molding cavity. Such a housing 104 confers additionalstructural strength to the circuit 100, protects the electronics fromcontaminants like moisture, dust particles, etc., and also protects theuser of possible harm. The housing also results in higher insulation andincreases the creepage and clearance compared to a similar case wherethe housing would not be present. For improved heat dissipation in highcurrent sensing applications, the entire leadframe, or only a portionthereof, may be exposed to a surrounding medium at the bottom of thehousing 104, e.g. may be exposed to air or to a heat sinking metal orpaste, and may additionally be made thicker to better resist the heatgenerated therein. However, the encapsulation of the circuit into ahousing 104 is only an optional feature of the invention and does nothave to be present in other embodiments.

The substrate 110 typically has a flat and thin body. The substrate maybe a semiconductor-on-insulator, for instance a silicon-on-insulatorsubstrate. The substrate may for instance have a thickness of less than12 μm, e.g. less than 10 μm, such as less than 6 μm. Of course, theactual thickness of the substrate 110 depends on the thickness of theburied insulation layer, hence of the intended application of thedevice. A thin substrate reduces the straight distance 109 between theat least one magnetic field sensing element 113 and the currentconductor 101, which increases the signal seen by the current sensingcircuit 100. In general, the substrate 110 is provided as asemiconductor substrate, e.g. a die cut from a wafer. For example,silicon or germanium substrates, or other suitable semiconductorsubstrates may be selected for the substrate 110. Providing thesubstrate 110 as a semiconductor die as the result of a wafer process isan attractive solution in terms of mass manufacturing capabilities,repeatability, reliability, and manufacturing costs. The substrate 110includes a first insulation layer 112, which is substantiallyincorporated into its body volume, and is typically required to have ahigh degree of purity, smooth material interfaces, and to be free ofdefects. Non-limiting examples of a buried, first insulation layer 112are nitride or oxide layers of a semiconductor material, e.g. a siliconoxide layer or a silicon nitride layer. A substrate material 111, 115 oneither side of the first insulation layer 112 may be the same material,e.g. crystalline silicon, or may be a different material or the samematerial in a different phase, e.g. the top silicon 115 may have astronger dopant concentration than the lower silicon 111, or the topsilicon 115 may be amorphous, whereas the lower silicon 111 iscrystalline. Moreover, the top surface of the upper substrate material115 is typically modified in some areas so as to implement electroniccircuits in it. The lower substrate material 111 may be thinned in someembodiments of the invention, for instance, if the substrate 110 is asemiconductor die of a wafer, the wafer may undergo a waferback-thinning process, e.g. by grinding and etching, so as to reduce thematerial thickness at a wafer back side, thus reducing the finalthickness of the semiconductor die. One example of a substrate 110including a first insulation layer 112, as shown in FIG. 1, may be agermanium-on-insulator or silicon-on-insulator substrate. An exemplarysilicon-on-insulator substrate may be obtained by a controlled oxygenion implantation process followed by an annealing step, by directre-growth of a thin silicon layer on an oxidized silicon layer usingseeding techniques, by wafer bonding techniques using, for instance,surface oxidized silicon wafers bonded onto a silicon handling wafer,etc. Therefore, the first insulation layer 112 may be implemented andsubstantially buried in a dielectric material that withstands highvoltages without breakdown, e.g. a silicon oxide layer. Particularlyminiaturized, e.g. small form factor integrated current sensing devices,are increasingly confronted with the problem of increased currentdensities which lead to elevated operative voltages (e.g. more than 1.5kV, e.g. at least 3 kV) under which the current sensing device still hasto function in a reliable and safe way. Even a thin first, substantiallyburied insulation layer 112 is providing a substantial degree ofinsulation of the current sensing circuit 100 with respect to the highoperating voltage ranges (e.g. up to 3 kV or more) in the currentconductor 101 if it is formed as a dielectric material layer with highelectrical breakdown voltage characteristics, e.g. a defect-free,crack-free, and uniform buried silicon oxide layer. Therefore, anelectrical breakdown voltage for the current sensing device isdetermined by the insulating properties of the first insulation layerwhich insulates from each other the first major surface 116 and thesecond major surface 117. An insulating property which is typicallyreported is the breakdown voltage, which, in the present case of asubstantially buried insulation layer, depends on the insulation layerdielectric material, density, quality, thickness, interfaces, etc.Besides, the first insulation layer 112 is efficiently blocking possibleleakage currents extending into the substrate 110 from its second majorsurface 117, e.g. its bottom side. Leakage currents may be caused byaging, temperature induced stresses, microcracks during packaging, theapplication of overrated voltages at the current conductor 101, etc.

In embodiments of the invention, the at least one magnetic field sensingelement 113 may be a sensing element based on the Hall-effect, e.g.planar or vertical Hall plates of variable shape, such as circular,wedged, polygonal, etc. The at least one magnetic field sensing element113 may be implemented in different materials, e.g. Si, Ge, GaAs,InGaAs, InSb, InAs, InP, other materials from the III-V compoundsemiconductor group, or any other suitable material. It could bedeposited by using micro-transfer printing techniques in someembodiments of the invention. Yet, alternative, non-limiting choices forthe at least one magnetic field sensing element 113 are offered bymagnetotransistors or magnetoresistive sensing elements exploiting thegiant magnetoresistance (GMR), the colossal magnetoresistance (CMR), thetunnel magnetoresistance (TMR), the extreme magnetoresistance (xMR), orthe anisotropic magnetoresistance (AMR) of some materials. One or moremagnetic field sensing elements 113 provided as a foreign material maybe transferred, e.g. by printing, or attached to the first major surface116 and electrically connected to electronic circuits also formed on thefirst major surface 116 of the substrate 110, e.g. on the first majorsurface 116 of a semiconductor substrate. Magnetic flux concentratinglayers may be locally applied in some embodiments of the invention.These flux concentrators are typically applied to the first majorsurface 116 of the substrate 110, in vicinity to the at least onemagnetic field sensing element 113 so as to locally concentrate themagnetic flux density in this region. This advantageously increases thesensitivity and signal-to-noise ratio of the current sensing circuit100. Magnetic flux concentrating layers, e.g. layers of ferromagneticmaterials, may also be used to redirect magnetic field lines in closeproximity to the at least one magnetic field sensing element 113 so asto optimize its relative position to the magnetic field lines generatedby the current conductor 101, and therefore optimize its responsivity,e.g. so as to have magnetic field lines traversing a planar Hall platesensing element in near ninety degree angles. More than one magneticfield sensing element 113 may be arranged on the first major surface116, for example two or three magnetic field sensing elements may beprovided, so as to enable differential measurements between almostidentical magnetic field sensing elements. This is advantageous forcalibration and accuracy of the current sensing circuit 100 as externalmagnetic fields, interfering with the generated magnetic field duringthe current measurement, can now be compensated for.

In operation, a current source, or a voltage source inducing a currentflow, is connected to the current conductor 101, e.g. via leadsbelonging to the primary portion of the leadframe. The current flowingthrough the current conductor 101 generates a constant or varyingmagnetic field, depending on whether the current flowing is a constantcurrent or an alternating current. This magnetic field is characterizedby magnetic field lines that wind in loops around the current conductor101, thus extending into regions of the substrate 110 away from thecurrent conductor 101. The at least one magnetic field sensing element113 detects the magnetic field generated and transduces it into anelectrical signal, e.g. a voltage signal, whose amplitude isproportional to the magnetic field strength under non-saturatingconditions. A transduced electrical signal may be directly output fromthe magnetic field sensing element 113, e.g. by directing it to one ormore leads of the secondary portion 102 of the leadframe via suitablebond pads 114, wire connections 105, etc., or may be processed beforebeing output. Processing steps may be carried out by suitable signalprocessing circuitry, which is also formed in the substrate 110, e.g.the semiconductor substrate, and which is adjacent to the magnetic fieldsensing element 113 to which it is connected. Pre-amplification,filtering, analog-to-digital conversion, modulation/demodulation,chopping, multiplexing/demultiplexing, sample/hold, averaging,correlated double sampling, output conversion to PWM signals,ratiometric signals, or encoded signal suitable for transmission inaccordance with a protocol, etc., all constitute examples of processingsteps.

FIG. 2 illustrates an embodiment of the invention which includes firstand second insulation layers 112, 212, not in direct contact with eachother and completely buried inside a substrate 210 of a circuit 200,e.g. an integrated sensor die. Substrate materials 111, 211, 115 oneither sides of the two insulation layers 211, 212 may be the same, e.g.a semiconductor material, e.g. silicon, or may differ, e.g. a siliconbottom and intermediate material 111, 211, and a germanium top material115, or may be similar but of different phases or doping levels. Thefirst and second insulation layers 112, 212 may be buried oxide ornitride layers of a substrate material, e.g. buried silicon oxide orsilicon nitride layers. As an example, two buried silicon oxide layersin a silicon substrate may be obtained by oxygen ion implantation onboth sides of the silicon substrate, i.e. on its first and second majorsurfaces 116, 117, followed by an annealing step. A straight distance209 from the current conductor 101 to the at least one magnetic fieldsensing element 113 of substrate 210 may be the same as the straightdistance 109 with regard to substrate 110, may be reduced, or may beincreased. A straight distance 209 may be reduced so as to increase themagnetic field strength at the at least one sensing element 113, andhence the sensitivity of the current sensing circuit 200. This reductionmay not negatively impact the insulating properties between the twomajor surfaces 116, 117 of the substrate 210, as the second insulationlayer 212 provides reinforced electrical insulation. For this particularembodiment, an electrical breakdown voltage for the current sensingdevice is not solely determined by the insulating properties (e.g.electrical breakdown voltage) of the first insulation layer 112, butalso by the insulating properties of the at least one second insulationlayer 212. As an example of reinforced electrical insulation, one mayconsider the case of partial discharges negatively affecting theinsulating properties of a substantially buried, second insulation layer212. The presence of the first insulation layer 112 then stillguarantees good electrical insulation with respect to a strong potentialdifference (e.g. 1.5 kV or more, e.g. up to 3 kV) between the twofunctional major surfaces 116, 117—one being coupled to the high voltagecurrent conductor 101, the other one carrying the sensitive low voltageelectronic circuits. A straight distance 209 may be increased forcurrent sensing applications for which the generated magnetic fieldstrength are well above a detection limit, e.g. sensing high electricalcurrents exceeding 10 A, e.g. currents up to 100 A (rms) and 150 A(peak). Then the reinforced insulation of the substrate 210 may beincreased by providing thicker insulation layers 112, 212, at theexpense of lowering the sensitivity of the integrated circuit by a smallamount. In all of the embodiments of the invention, an additionalexternal insulation layer may be applied between the current conductor101 and the substrate 110, 210 for obtaining supplementary insulation,e.g. an externally applied insulation layer comprised by an attachmentlayer 103, e.g. an insulating tape. A particular advantage of providingtwo insulation layers 112, 212 and an intermediate substrate material211 placed between them is that the intermediate substrate material 211can be electrically grounded, e.g. by TSV connections providingconductive channels through one or both insulation layers. This meansthat, even in the event of failure of the second insulation layer 212,leaking charges that accumulate at an interface between the intermediatesubstrate material 211 and the first insulation layer 112 can beefficiently removed whereby a risk of electrical shock or damage of thecurrent sensing circuit 200 is greatly reduced.

FIG. 3 is a schematic perspective view of a part of a current sensingcircuit, e.g. an integrated circuit, and FIG. 4 is a schematiccross-section (along the line IV-IV) thereof. The numerous detailsbrought forth in FIG. 3 and FIG. 4 have already been described inrespect of foregoing embodiments, except for a protruding section 106 ofthe current conductor 101 and for a cavity 107 formed at the secondmajor surface 117 of the substrate. Forming a cavity 107 in vicinity ofthe at least one magnetic field sensing element 113 has the advantage oflocally thinning the substrate whereby a straight distance 109, 209between the current conductor 101 and the at least one magnetic fieldsensing element 113 is reduced, which increases the signal seen by thecurrent sensing integrated circuit. An alternative way of thinning thesubstrate and reducing a straight distance 109, 209 between the currentconductor 101 and the at least one magnetic field sensing element 113 isthe partial, removal of substrate material 111 at the second majorsurface 117 of the substrate, e.g. by grinding, polishing, etching, etc.For embodiments in which the substrate is provided as a semiconductordie, this may be achieved in a wafer back side thinning process. Theformation of the cavity 107 has the advantage that the reduction of asubstrate thickness is only obtained locally, whereas the remainder ofthe substrate is unaffected and ensures the structural integrity of thesubstrate as a whole, e.g. avoids breaking or cracking of the alreadythin substrate during handling, assembly, and stress tests. In someembodiments of the invention, the cavity 107 is formed such that the atleast one first insulation layer 112 is partially exposed at the bottomof the cavity 107 and accessible through the second major surface 117.Yet, the first insulation layer 112 is still substantially buried in theremaining volume of the substrate. In cases for which there is more thanone, e.g. more than the first insulation layer 112 in the substrate,i.e. there exists at least a second insulation layer 212, the insulationlayer which is located closest to the second major surface 117 may bepartially exposed at the bottom of the cavity 107. In other embodiments,the cavity 107 does not reach all the way through the substrate material111 such that a substantially buried insulation layer is not exposed,e.g. forms a completely buried insulation layer. For some embodiments ofthe invention, a cavity 107 may be obtained at the second major surface117 by chemical etching an opening into the substrate material 111, e.g.by dry etching, wet etching, plasma etching, etc. If a dry etchingprocess is used to form the cavity 107, very steep cavity side wallsthat are substantially forming ninety-degree angles with the secondmajor surface 117 can be obtained.

In preferred embodiments of the invention, the current conductor 101 isformed to have a flat section, e.g. as a flat stripe, extending in aplane parallel to the second major surface 117 whereby a good couplingto the second major surface 117 can be obtained. The protruding section106 is projecting out of a plane defined by the flat section of thecurrent conductor 101 and is positioned with respect to the substratesuch that it extends into the cavity 107. This configuration of thecurrent conductor 101 allows to position the current conductor 101 at aneven closer straight distance to the at least one magnetic field sensingelement 113. This increases the sensitivity of the integrated circuitwithout removing or thinning an insulation layer. It also facilitatesthe positioning of the current conductor 101 with respect to the atleast one magnetic field sensing element 113. For a better responsivityand sensitivity of the current sensing circuit, and less offsetcompensation requirements due to positional asymmetries between thecurrent conductor 101 and one or more magnetic field sensing elements113, a very accurate and precise positioning of the current conductor101 during assembly of the current sensing circuit, e.g. at the time ofcoupling the current conductor 101 to the substrate, is mandatory.Embodiments of the invention which include the cavity 107 and aprotruding section 106 have the advantage that the protruding section106 may be easily locked into the cavity 107 as long as the relativedimensions are selected accordingly.

In some embodiments of the invention, the protruding section 106 of thecurrent conductor 101 may be obtained by means of a stamping tool. Inalternative embodiments, the protruding section 106 of the currentconductor 101 may be obtained by means of etching an initially thickleadframe, or a thick primary portion, everywhere except for a portionprotected from etching which corresponds to the protruding section 106after finishing the etch.

At the time the integrated circuit is assembled, a thin layer ofconductive paste 108 may be provided between the current conductor 101and the second major surface 117 for some embodiments of the invention,as shown, for instance, in FIG. 4. The conductive paste may be part ofthe attachment layer 103. A thin layer of conductive paste 108 mayimprove the overall adhesion of the current conductor 101 with respectto the substrate, and it may also conduct excess heat away from thecurrent conductor. Additionally, a more uniform current flow at atargeted straight distance between the current conductor 101 and the atleast one magnetic field sensing element 113 may be the result of asmoother attachment interface, e.g. by smoothing out surface defects androughness, or non-planar surface portions of the current conductor 101.Therefore, a current can be measured more accurately by the currentsensing circuit.

FIG. 5 illustrates a cross-section that is similar to the one shown inFIG. 4, but which has chamfered cavity 107 side walls. A cavity 107having chamfered side walls has a bigger cross-sectional area at theside of the second major surface 117 and a smaller cross-sectional areaat its bottom part. This may facilitate even more the introduction ofthe protruding section 106 into the cavity 107. Furthermore, it is anadvantage of a cavity 107 having chamfered side walls that the substratecan rest on and being supported by the primary portion of the leadframe,by virtue of the protruding section 106 abutting on the chamfered sidewalls of the cavity 107, as illustrated in FIG. 5, without damaging theinsulation layer, or more generally damaging the thin membranecomprising thinned substrate material 111, the first insulation layer112, and other substrate material 115. Damaging the first insulationlayer 112 would negatively affect the insulation strength of thesubstrate and the current sensing circuit (e.g. integrated currentsensor) as a whole. In other embodiments of the invention, the substratemay rest on and be supported by the flat section of the currentconductor 101, as depicted in FIG. 4, thereby providing a larger andmore robust support area.

In some embodiments of the invention, the cavity 107 may be partially orcompletely filled by a low partial discharge susceptible silicone gelbefore the substrate is physically coupled, e.g. attached by anattachment layer 103, to the current conductor 101. This is protectingthe first insulation layer 112 because silicone acts as an insulator andsealing agent at the same time, preventing moisture and reactants fromslowly degrading the one or more substantially buried insulation layers.In a synergic effect, silicone gel also provides good thermal andelectrical insulation, and is a good adhesive. Temperature fluctuationsat the at least one magnetic field sensing element 113 caused by aheating of the current conductor 101 may require regular re-calibrationof the (integrated) current sensing circuit to not impact the accuracyof the current measurement, and therefore, may be avoided by providinggood heat sinking means and good thermal insulation of the at the atleast one magnetic field sensing element 113. In this respect, the firstinsulation layer 112, e.g. a buried silicon dioxide layer, may alsoprovide good thermal insulation means.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive. Theforegoing description details certain embodiments of the invention. Itwill be appreciated, however, that no matter how detailed the foregoingappears in text, the invention may be practiced in many ways. Theinvention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure and the appendedclaims. In the claims, the indefinite article “a” or “an” does notexclude a plurality. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measures cannot be used to advantage.

1. A circuit for sensing current, the circuit comprising: a substratehaving a first and a second major surface, the second major surfacebeing opposite to the first major surface; at least one magnetic fieldsensing element arranged on said first major surface of the substrateand suitable for sensing a magnetic field caused by a current flow in acurrent conductor coupled to the second major surface; wherein thesubstrate comprises a first insulation layer positioned between thefirst major surface and the second major surface such that it issubstantially buried in the substrate.
 2. The circuit according to claim1, wherein the substrate is a semiconductor-insulator-semiconductorsubstrate, wherein the insulator forms the first insulation layer. 3.The circuit according to claim 1, further comprising an insulating tapecoupled to the second major surface.
 4. The circuit according to claim1, further comprising a leadframe comprising a primary and a secondaryportion, the primary and secondary portion each comprising a pluralityof leads, wherein at least two of the leads of the primary portion ofthe leadframe are electrically connected to each other to form a currentconductor.
 5. The circuit according to claim 1, wherein a thickness ofthe substrate is reduced in the vicinity of the at least one magneticfield sensing element.
 6. The circuit according to claim 5, wherein acavity is formed at the second major surface and in the vicinity of theat least one magnetic field sensing element.
 7. The circuit according toclaim 6, wherein the cavity comprises chamfered side walls and/orstraight side walls.
 8. The circuit according to claim 6, furthercomprising a leadframe comprising a primary and a secondary portion, theprimary and secondary portion each comprising a plurality of leads,wherein at least two of the leads of the primary portion of theleadframe are electrically connected to each other to form a currentconductor; wherein the current conductor comprises a flat section and aprotruding section, the flat section extending in a plane parallel tothe second major surface, the protruding section extending out of saidplane and into the cavity.
 9. The circuit according to claim 4, whereinthe substrate is resting only on the primary portion of the leadframeand does not overlap the secondary portion of the leadframe in a planview parallel to the first major surface.
 10. The circuit according toclaim 1, wherein the substrate further comprises at least one secondinsulation layer positioned between the first major surface and thesecond major surface such that it is substantially buried in thesubstrate and not touching the first insulation layer.
 11. The circuitaccording to claim 10, wherein a substrate material located between thefirst insulation layer and the second insulation layer is electricallygrounded.
 12. A method of manufacturing a device for measuring acurrent, the method comprising: providing a semiconductor substratehaving a first and a second major surface, the second major surfacebeing opposite to said first major surface, and a first insulation layerwhich is, over at least part of its surface, buried in the substratebetween said first and second major surface; and providing at least onemagnetic field sensing element on the first major surface, the magneticfield sensing element being suitable for sensing current flowing througha current conductor physically coupled to the second major surface. 13.The method according to claim 12, further comprising providing a currentconductor and physically coupling the current conductor to the secondmajor surface.
 14. The method according to claim 13, wherein providing acurrent conductor comprises: providing a flat leadframe including aplurality of leads; connecting at least two leads of the plurality ofleads so as to form said current conductor as a flat current conductor;and etching a portion of the flat current conductor and/or using astamping tool to form a protruding section along the flat currentconductor.
 15. The method according to claim 12, further comprising thestep of etching a cavity into the semiconductor substrate, the cavitybeing etched at the second major surface and in the vicinity of the atleast one magnetic field sensing element.